| 3650605 | INTERFEROMETRIC APPARATUS WITH CONTROLLED SCANNING MEANS | Little, Jr. | 359/577 | |
| 4093338 | Apparatus for piecewise generation of grating-like patterns | Bjorklund et al. | 359/35 | |
| 5066133 | Extended length embedded Bragg grating manufacturing method and arrangement | Brienza | 359/570 | |
| 5363239 | Method for forming spatially-varying distributed Bragg reflectors in optical media | Mizrahi et al. | 359/569 | |
| 5367588 | Method of fabricating Bragg gratings using a silica glass phase grating mask and mask used by same | Hill et al. | 385/37 | |
| 5604829 | Optical waveguide with diffraction grating and method of forming the same | Bruesselbach | 385/37 | |
| 5655040 | Method of forming a dispersing grating in an optical fiber | Chesnoy et al. | 385/37 | |
| 5748814 | Method for spatially controlling the period and amplitude of Bragg filters | Painchaud et al. | 385/37 |
| EP0631158 | Method for forming distributed bragg reflectors with spezially varying amplitude in optical media. | |||
| GB2212935 | ||||
| WO/1995/022068 | OPTICAL GRATING | |||
| WO/1996/024079 | BROADBAND GRATING | |||
| WO/1996/036895 | OPTICAL WAVEGUIDE GRATING |
This invention relates to fabricating optical waveguide gratings.
Dispersion compensation is an attractive technique allowing the upgrade of the existing installed standard fibre network to operation at 1.5 μm where it exhibits a dispersion of˜(about) 17 ps/nm.km which would otherwise prohibit high capacity (eg. 10 Gbit/s) data transmission.
Chirped fibre gratings are currently the most attractive technique for fibre dispersion compensation [1]. This is because they are generally low loss, compact, polarisation insensitive devices which do not tend to suffer from optical non-linearity which is the case with the main competing technology, dispersion compensating fibre.
For present practical applications chirped gratings must exhibit both high dispersion, ˜1700 ps/nm, sufficient to compensate the dispersion of around 100 km of standard fibre at a wavelength of 1.55 μm, and a bandwidth of around 5 nm. This implies a need for a chirped grating of length 1 m.
Fibre gratings are generally created by exposing the core of an optical fibre to a periodic UV intensity pattern [2]. This is typically established using either an interferometer or a phase mask [3]. To date, phase masks are the preferred approach owing to the stability of the interference pattern that they produce. The length of the grating can be increased by placing the fibre behind the phase mask and scanning the UV beam along it. Techniques for post chirping a linear grating after fabrication include applying either a strain [1] or temperature gradient [4] to it. However these techniques are limited due to the length of the initial grating (˜10 cm with available phase masks) and the length over which a linear temperature or strain gradient can be applied. Alternatively more complex step chirped phase masks can be employed [5]. However, all of these techniques are currently limited to a grating length of about 10 cm.
In addition to chirping the grating, it is also sometimes desirable to be able to apodise (window) the gratings to reduce multiple reflections within them and to improve the linearity of the time delay characteristics. A powerful technique has been developed which allows chirped and apodised gratings to be written directly in a fibre, referred to as “the moving fibre/phase mask scanning beam technique” [6]. This technique is based on inducing phase shifts between the phase mask and the fibre as the phase mask and fibre are scanned with the UV beam. Apodisation is achieved by dithering the relative phase between the two at the edges of the grating. Like all the previous techniques the one draw back with this technique is that it is again limited to gratings the length of available phase masks, ˜10 cm at present.
This problem has been overcome in one approach by Kashyap et al using several 10 cm step-chirped phase masks [5]. These are scanned in series to obtain a longer grating. The phase “glitch” or discontinuity between the sections is subsequently UV “trimmed” to minimise its impact. However this is a time consuming and costly process. In addition the effect of the UV trimming will vary with grating ageing.
A technique for potentially writing longer gratings has been reported by Stubbe et al [7]. In this case a fibre is mounted on an air-bearing stage and continuously moved behind a stationary grating writing interferometer. The position of the fibre is continuously monitored with a linear interferometer. The UV laser is pulsed to write groups of grating lines with period defined by the writing interferometer. A long grating can be written by writing several groups of grating lines in a linearly adjacent series, with controlled phase between the sections. The phase shift between each group of grating lines is controlled via the linear interferometer and a computer which sets the time the laser pulses. A short pulse, ˜10 ns, is required such that the position of the writing lines is effectively stationary and accurately controlled with respect to fibre motion. Having said this, however, jitter in the pulse timing and in the linear interferometer position will give detrimental random phase errors in the grating. Chirped gratings can potentially be fabricated by continuously introducing phase shifts between adjacent groups along the grating. Obviously the maximum translation speed is limited by the number of grating lines written with one laser pulse and the maximum repetition rate of the pulsed laser. It is also proposed in this paper that apodisation is achieved by multiple writing scans of the grating.
This invention provides a method of fabricating an optical waveguide grating having a plurality of grating lines of refractive index variation, the method comprising the steps of:
(i) repeatedly exposing a spatially periodic writing light pattern onto a photosensitive optical waveguide; and
(ii) moving the writing light pattern and/or the waveguide between successive exposures or groups of exposures of the writing light pattern, characterised in that
the successive exposures or groups of exposures overlap so that each of at least a majority of the grating lines is generated by at least two exposures to different respective regions of the writing light pattern.
Embodiments of the invention provide a number of advantages over previous techniques:
1. The realisation that the laser does not have to be pulsed but just has to be on for a particular duty cycle—preferably less than 50% of the period. This allows an externally modulated CW (continuous wave) laser to be used.
2. With this technique the grating lines are re-written by several successive exposures of the writing light beam at every grating period (or integral number of grating periods). Thus the footprint defined by the writing light beam is significantly overlapped with the previous lines. Significant averaging of the writing process is achieved thus improving the effective accuracy and resolution of the system, compared to that of [7] where a group of lines is written in a single exposure, and the fibre is then advanced to a fresh portion where a further group of lines is written in a single exposure.
3. Effectively controlling the grating writing process on a line-by-line basis allows accurate apodisation to be achieved. This may be performed in embodiments of the invention by dithering the grating writing interferometer position in the fibre to wash out or attenuate the grating strength whilst keeping the average index change constant.
4. The technique offers the further advantage that the CW laser may be-extremely stable, whereas pulsed lasers (e.g. those used in [7]) may suffer from pulse-to-pulse instability which is not averaged. In addition the high peak powers of the pulsed laser may cause non-linear grating writing effects.
5. Arbitrary phase profiles and in particular a linear chirp can be built up by inducing phase shifts electronically along the grating as it grows. In a similar manner to the “Moving fibre/phase mask” technique [6] the maximum wavelength is inversely proportional to the beam diameter. This can be further improved in particular embodiments of the invention by incorporating a short, linearly chirped phase mask, Thus as the fibre is scanned the UV beam may be also slowly scanned across the phase mask, an additional small phase shift is induced, whilst most significantly we have access to writing lines of a different period allowing larger chirps to be built up
This invention also provides apparatus for fabricating an optical fibre grating having a plurality of grating lines of refractive index variation, the apparatus comprising:
a writing light beam source for repeatedly exposing a spatially periodic writing light pattern onto a photosensitive optical waveguide; and
means for moving the writing light pattern and/or the waveguide between successive exposures or groups of exposures of the writing light pattern, characterised in that
the successive exposures or groups of exposures overlap so that each of at least a majority of the grating lines is generated by at least two exposures to different respective regions of the writing light pattern.
The various sub-features defined here are equally applicable to each aspect of the present invention.
The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:
The fibre is continuously and steadily linearly translated or scanned in a substantially longitudinal fibre direction during the grating exposure process.
Ultraviolet (UV) light at a wavelength of 244 nm from a Coherent FRED laser
The relative position of the fibre to the interference pattern of the phase mask is continuously monitored with a Zygo, ZMI1000 differential interferometer
So, as each position value is output by the interferometer, the controller
If the fibre was scanned with the UV beam continuously directed onto the fibre, no grating would be written since the grating lines would be washed out by the movement.
However if the UV beam is strobed or modulated (under control of the switching position data generated by the computer
then a long grating would grow.
This expression is based on a time period of a temporally regular modulation of the UV beam, and so assumes that the fibre is translated at a constant velocity by the translation stage. However, more generally, the switching on and off of the UV beam is in fact related to the longitudinal position of the fibre, so that in order to generate a grating the UV beam should be turned on and off as the fibre is translated to align the interference pattern arising from successive exposures through the phase mask.
In
The UV beam is modulated by the acousto-optic modulator in a periodic fashion synchronised with the translation of the fibre. In this way, successive exposures, such as the two subsequent exposures shown in
Although each of the successive exposures of the fibre to UV light through the phase mask
As the duty cycle for the UV exposure increases, the grating contrast decreases (because of motion of the fibre during the exposure) but the writing efficiency increases (because more optical energy is delivered to the fibre per exposure). Thus, selection of the duty cycle to be used is a balance between these two requirements.
Assuming linear growth, the index modulation, n
where z is the position down the fibre and Λ the grating period. With the new technique we obtain:
where ΔΛ
For small values of ΔΛ
The maximum grating strength is obtained for ΔΛ
Experimentally, a good value for ΔΛ
Thus, with embodiments of this technique, exposure of the grating lines or elements is repeated every grating period. Thus the footprint defined by the UV beam, which might for example for a 500 μm diameter beam, φ
The computer in this embodiment actually generates the switching positions internally as “real” numbers (obviously subject to the limitation of the number of bits used), but then converts them for output to the controller into the same unit system as that output by the Zygo interferometer, namely multiples of a “Zygo unit” of 1.24 nm. This internal conversion by the computer makes the comparison of the actual position and the required switching position much easier and therefore quicker for the controller. A random digitisation routine is employed in the computer
The technique offers the further advantage that the CW laser is extremely stable whereas pulsed lasers (as required in the technique proposed by Stubbe et al [7]) may suffer from pulse-to-pulse instability which, in the Stubbe et al technique, is not averaged over multiple exposures. In addition the high peak powers of a pulsed laser may cause non-linear grating writing effects, which are avoided or alleviated by using longer and repeated exposures in the present technique.
A refinement of the above technique, for producing apodised gratings, will now be described with reference to
Using the techniques described above, effectively controlling the grating writing process on a line-by-line basis allows accurate apodisation to be achieved.
Apodisation is achieved by effectively dithering the grating writing interferometer position in the fibre to wash out or attenuate the grating strength. However, if the overall duty cycle of the exposure is kept the same, and just the timing of each exposure dithered, the average index change along the grating is kept constant.
To completely wash out the grating subsequent on periods of the UV laser are shifted in phase (position) by ±π/2(±Λ/4). To achieve a reduced attenuation the amplitude or amount of dither is reduced.
This technique of apodising is better with an exposure duty cycle of less than 50%, to allow a timing margin for 100% apodisation.
One example of the use of this technique is to generate a grating with a contrast increasing at one end of the grating according to a raised cosine envelope, and decreasing at the other end of the grating in accordance with a similar raised cosine envelope, and remaining substantially constant along the central section of the grating. This apodisation can be achieved particularly easily with the present technique, as the central section requires no phase shift between successive exposures, and the two raised cosine envelopes require a phase shift that varies linearly with longitudinal position of the fibre.
The required phase shifts can be calculated straightforwardly by the computer
Other apodisation schemes are also possible. Compared with previous methods of dithering [6] this technique is not limited by the dynamics of a mechanical stage used for dithering, but instead simply adjusts the switching time of a non-mechanical modulator element
Furthermore, arbitrary phase profiles and in particular a linear chirp can be built up by the computer
In particular, therefore,
A ˜4 nm bandwidth and dispersion of ˜5000 ps/nm are observed.
Such results have not been reported by any other method. Gratings up to 40 cm and writing speeds up to 1 mm/s have been demonstrated. Lengths in excess of 1 cm and writing speeds up to 10 mm/s are feasible.
In the above description, the fibre has been translated with respect to the phase mask, and in the later description the UV beam is translated with respect to the phase mask. However, it will be clear that the important thing is relative motion, and so the choice of which component (if ants) remains “fixed” and which is translated is relatively arbitrary. Having said this, however, the arrangement described above has been tested experimentally and has been found to be advantageously convenient to implement. It will also be apparent that in other embodiments each “exposure” could in fact involve a group of two or more exposures, with the position of the fibre with respect to the writing light beam being constant or substantially constant for exposures within a group, but different from group to group.
1. D. Garthe et al, Proc. ECOC, vol. 4, (post-deadline papers), pp. 11-14 (1994).
2. G. Meltz et al, Opt. Lett., 14(15), pp. 823-825, 1989.
3. K. O. Hill et al, Appl. Phys. Lett., 62(10), pp. 1035-1037, 1993.
4. R. I. Laming et al, Proc.ECOC'95, Brussels, Vol 2, Paper We.B.1.7, pp 585-8, Sep. 17-21, 1995.
5. R. Kashyap et al, Electronics Letters, Vol 32 (15), pp. 1394-6, 1996.
6. M. J. Cole et al, Electronics Letters, Vol 31 (17), pp 1488-9, 1995.
7. R. Stubbe et al, postdeadline paper 1, Proc. Photosensitivity and Quadratic Nonlinearity in Glass Waveguides, Portland, Oreg., Sep. 9-11, 1995.